This invention relates to the field of wireless communication, and more specifically to the field of radio wave antennas. This invention provides planar inverted-F antennas (PIFAs) for use in wireless communication devices (WCDs) such as cellular wireless devices and wireless personal communication devices, wherein the PIFAs include a matching network.
It is known that a WCD may include a PIFA having a matching network.
For example, US published patent application U.S. Ser. No. 2001/0033250 A1 (incorporated herein by reference) describes an asymmetrical dipole antenna having a planar ground plane element, a three-fingered matching network, and a resonator element, the antenna being adapted to fit within the housing of a WCD. The resonator element is closely spaced and generally parallel to the matching network and the underlying ground plane element. Skirt portions of the resonator element are folded downward toward the matching network. A first conductor extends downward to connect the resonator element to the ground plane element. A second conductor extends downward to connect the resonator element to the matching network. A third conductor extends downward to connect the central-finger of the three-finger matching network to the ground plane element. The resonator element includes a serpentine conductor having two physically spaced open-ends, having a first conductor-portion that resonates within the cell phone band of 880-960 MHz, and having a second conductor-portion that resonates within the personal communications services (PCS) band of 1710-1880 MHz. An optional tuning capacitor is connected between one of the two open-ends and the ground plane element. A 50 ohm feed-point for the antenna is located at one of the two outside fingers of the three-finger matching network. The central finger of the matching network is in the nature of a matching stub, and the other outside finger of the three-finger matching network is in the nature of a series resonant matching element.
It is desirable that the antenna of a WCD simultaneously function across multiple frequency bands, and that these frequency bands be wide frequency bands. It is also desirable that the antenna be of a small physical size, so as to be unobtrusive, and so as to enable a pleasing industrial design to be provided for the WCD.
As used herein the term bandwidth can be defined as the width of a communications channel. In analog communications, bandwidth is typically measured in cycles per second (Hertz). In digital communications, bandwidth is typically measured in bits per second (bps). It is often desired that these bandwidths be wide bandwidths. That is that the range of frequencies over which power is transferred to, and received from, the WCD""s antenna be wide.
PIFAs are well suited for use as WCD embedded antennas, and PIFAs can provide a good match at different frequencies simultaneously, without the need for a matching network, thus providing multi-band operation. However, when the frequency bands are close together, or wide, matching becomes more difficult.
It is also known that as the physical volume that is enclosed by a PIFA decreases, the PIFA""s bandwidth of operation decreases. Thus, a typical PIFA will reach limits in bandwidth as the physical size of the PIFA is reduced. For example, a typical PBW of a small size dual-band PIFA (for example 880-960 MHz and 1710-1880 MHz) used in hand-held communications devices is about 10 percent, wherein PBW can be defined as 100 times the upper frequency of the bandwidth minus the lower frequency of the bandwidth divided by the square root of the upper frequency of the bandwidth times the lower frequency of the bandwidth.
Matching networks have been used to reduce power that is reflected from an antenna""s input, thus allowing the antenna to operate over a wider bandwidth.
When a matching network includes discrete electrical components or discrete circuit elements to provide additional poles (singularities) to the matching network""s transfer function, each positive frequency pole typically requires the addition of two discrete electrical components, thus increasing the cost and reducing the reliability of the antenna.
Distributed matching networks that are made up of microstrip transmission lines inherently provide multiple poles and zeros within the transfer function of the matching network. However, because distributed matching networks are often on the order of a wavelength in physical size, such matching networks can require a large physical area, especially when such matching networks are used to match multiple bandwidths.
A common technique to provide wideband matching is to use shorted and open transmission line stubs in parallel (for example, see MICROWAVE CIRCUIT DESIGN, John Wiley and Sons, 1990, at pages 180-181).
Transmission line stubs are distributed circuits, and by adjusting the physical parameters of the stubs it is often possible to place zeros to cancel undesirable poles and to add other poles at more beneficial frequencies. However, the problem of using this technique in multi-band antenna designs is that while one frequency band widens due to a match that is achieved by the use of transmission line stubs, another frequency band is corrupted due to the addition of the transmission line stubs.
This invention provides a dual-band PIFA having a unique matching network that is incorporated into a unique physical position within the PIFA using a one or more unique manufacturing process steps. The matching network selectively tunes the PIFA to at least two desired frequency bands, and the matching network intrinsically provides a good match in the frequency bands that are of interest.
When the frequency bands of interest do not have a desired bandwidth, a microstrip stub technique is used to widen the bandwidth for these frequency bands.
In accordance with the invention, and using one or more discrete-component LC tank circuits, one or more microstrip stubs are high-impedance-disconnected from the matching network at one or more frequency bands wherein it is not desired have these microstrip stubs operate. As a result, the invention eliminates the need to provide additional microstrip stubs or other components in order to achieve matching over multiple frequency bands that have wide bandwidths.
An embodiment of this invention provides a dual-band PIFA having a small-size matching network that is integrated into the PIFA, wherein the PIFA includes a metallic radiating/receiving element (hereinafter radiating element) and a metallic ground plane element. As a result of this new and unusual construction and arrangement a PIFA and its matching network is provided within a physical volume that is no larger than the physical volume that is required for the basic components of a PIFA.
In accordance with a feature of the invention, the matching network includes at least one discrete capacitor (C) component, at least one discrete inductor (L) component, and distributed microstrip transmission line stubs that cooperate to broadband/wideband match to the antenna""s radiating element within at least two frequency bands.
In addition, the antenna and its integral matching network are manufactured as a single physical part, to thus form a single unitary assembly for mounting on a main printed circuit board (PCB) of a WCD. One utility of the invention is for use within small mobile telephones that can be carried in a shirt pocket.
In a non-limiting embodiment of the invention the distributed transmission-line portion of the matching network included an antenna-feed transmission line stub that was connected to the antenna""s radiating element, a radio-feed transmission line stub that was connected to the input of a WCD, a shorted transmission line stub, and an open transmission line stub.
In this embodiment of the invention the open transmission line stub was effectively disconnected from the matching network at the lower frequency band by connecting a parallel LC frequency trap (i.e. a discrete-component LC tank circuit) in series with the open transmission line stub. This LC trap was formed by the parallel connection of a discrete capacitor and a discrete inductor, and the LC trap was tuned to resonate at a frequency that was at, or near to, the center frequency of the low frequency band.
While optimized performance of this embodiment of the matching network can place the resonant frequency of the LC trap away from the center frequency of the low frequency band, this resonant frequency is usually closer to the low frequency band than it is to the high frequency band.
This LC trap became a high impedance at the low resonant frequency of the LC trap, and this high impedance effectively disconnected the open transmission line stud from the matching circuit for frequencies in the low frequency band, thus mitigating the effects of the open transmission line stub on a match to the low frequency band, which match was optimized in this embodiment by the shorted transmission line stub and by the physical structure of the antenna""s radiating element.
While the above-described embodiment of the invention provided that an LC trap was connected in series with only the open transmission line stub, within the sprit and scope of the invention an LC trap can be connected in series with only the shorted transmission line stub, or an LC trap can be connected in series with both of the open transmission line stub and the shorted transmission line stub.
That is, within the scope of this invention a matching network is provided having open and shorted transmission line stubs and LC traps, to thereby form a matching network that matches an antenna""s radiating element to the input of a radio device such as a transmit/receive WCD within at least two frequency bands.
Because matching networks in accordance with the invention include one or more discrete-component LC tank circuits that operate to selectively disconnect one or more transmission line stubs at one or more desired frequency bands, the use of long transmission lines, and the use of a large number of discrete circuit components, is avoided.
In the above-described embodiment of the invention the high frequency band was from about 1710 MHz to about 2170 MHz, this corresponding to a PBW of about 24 percent.
A small physical volume for the PIFA is achieved in accordance with the invention both by a unique configuration of the matching network and by integrating the matching network directly under the antenna""s radiating element. By integrating the matching network directly under the antenna""s radiating element the size-footprint of the PIFA no larger than the size-footprint of the PIFA itself, this usually being the size of the antenna""s ground plane element.
In addition, low cost is achieved in accordance with the invention by forming the matching network and other portions of the PIFA using one of two manufacturing process, i.e. by using (1) a stamped/bent metal process wherein the discrete LC components and an antenna feed are soldered onto a stamped/bent metal part, and wherein the resulting assembly is then surface-mounted onto an input/output WCD feed that is carried by the ground plane element and the main PCB of the WCD, or by using (2) a two-shot molding process wherein the discrete components are soldered onto a selectively-metallized two-shot molded assembly, and wherein the resulting assembly is then surface-mounted onto an input/output WCD feed that is carried by the ground plane element and the main PCB of the WCD, wherein the later process is a preferred process.
In an embodiment of the matching network""s transmission line portion, the matching network""s transmission line stubs, and the antenna""s radiating element were made of a common electrically conductive material.
In addition, the dielectric substrate that carries the matching network""s transmission line portion, the matching network""s transmission line stubs, and the antenna""s radiating element can comprise a common dielectric member.
In summary, and in accordance with the present invention, a multi-band antenna is impedance-matched to a multi-band wireless communications device by providing a microstrip transmission line that connects the antenna to the wireless communications device. A plurality of microstrip stubs are connected to the microstrip transmission line, and one or more LC tank circuits are associated with the microstrip stubs to selectively disconnect certain of the microstrip stubs from the microstrip transmission line in a manner to provide impedance matching within each of the multiple bands.